The present invention relates to fabricating a circuitized substrate having through-holes. More particularly, the present invention relates to a method for masking through-holes in a substrate during the circuitization or plating of the substrate.
In the manufacture of printed circuit boards and cards, a dielectric sheet material is employed as the substrate. A conductive circuit pattern is provided on one or both of the major surfaces on the substrate. Generally speaking, high density printed circuit boards are constructed with several electrically conductive layers separated by dielectric layers. The dielectric layers typically contain through-holes for the purpose of making interconnections between electrically conductive layers. The holes are plated with an electrically conductive material such as copper.
When selectively plating the dielectric substrate to provide the circuitization, a plating resist is provided on the substrate to protect those areas which are not to be plated at this stage of the process and which typically includes the already plated through-holes. Accordingly, it necessary to tent the holes with the resist material during the circuitization.
However, various plating compositions cause certain problems in this regard. By way of example, electroless plating of gold usually involves relatively caustic compositions (high pH) and plating conditions employing relatively high temperatures. Due to the caustic nature of typical electroless gold plating chemistry, the available plating resists capable of withstanding such conditions are rather limited. Those resists of choice although quite capable of withstanding the plating conditions, do not exhibit especially good tenting capability and especially tenting characteristics needed to tent relatively large holes ( greater than 30 mils) that are generally present on multilayer circuit boards. Accordingly, using such creates a risk of broken tents over the holes. This in turn results in subsequent plating of relatively thick electroless gold in the plated through-holes. The presence of such gold in the placed through-holes represents a significant reliability concern since the gold is capable of interacting with solder employed in subsequent assembly processing. This in turn can create an intermetallic which would thereby weaken the reliability of the solder joint.
Accordingly, it would be desirable to provide a process that makes it possible to circuitize a substrate having at least one hole therein and to tent the hole successfully regardless of the plating conditions and plating resist employed.
The present invention provides a process which makes it possible to successfully tent holes on a substrate during the circuitization of the substrate even when using plating resist compositions that do not exhibit very good tenting characteristics such as the available resists for electroless gold plating. More particularly, the present invention provides a method for masking a hole in a substrate during plating of the substrate by providing a substrate with first and second external surfaces and having at least one hole extending substantially through the substrate and then subsequently filling the at least one hole with a liquid material. A photoimageable material is then applied to at least one of the external surfaces of the substrate followed by forming a predetermined pattern in the photoimageable material and then circuitizing the predetermined pattern by plating a conductive metal onto the substrate. The photoimageable material is then removed from the at least one external surface of the substrate followed by removal of the liquid material from the at least one hole in the substrate.
The presence of the filled holes provide a base for the photoimageable material to adhere to over the holes of the substrate thereby eliminating the risk of broken tents.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described only the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.